1. Field of the Invention
This invention relates to digital recording media and more particularly to high capacity recording media for optical recording and read-out.
2. Description of the Related Art
Digital information has been recorded on many kinds of materials by a wide variety of processes. One widely used system magnetizes selected areas of a disk of magnetic material to represent the information to be preserved. Other approaches, used primarily when greater recording density is desired, include the use of a substrate coated with a material capable of being changed by selective treatment with a laser beam. For example, the substrate may have a reflective surface that is caused to have lower reflectivity in those areas where it is heated by a focused laser beam. Conversely, non-reflecting absorbent surfaces have been rendered reflective by the application of a laser beam as described in Optical Memory News, September-October 1984, page 14. The reflectivity of the surface may be altered by melting or deforming the surface or by actual evaporation of material from the surface. Most often, the medium is in the form of a disk and the information is recorded along a spiral track. The timing and tracking information may also be recorded by a modulated laser beam following the spiral track.
U.S. Pat. Nos. 4,214,249 to Kasai; 4,270,916 to Dil; 4,379,299 to Fitzpatrick et al.; 4,314,262 to Reilly; and 4,334,299 to Komurasaki et al. disclose media of the kinds referred to above.
The Dil Patent discloses recording on a disk having a grooved spiral and in which timing marks are recorded on the sloping walls of the grooves.
The Kasai patent discloses the recording of digital information by the selective exposure to a laser beam that causes deformation or evaporation of a layer composed of S, Se, Te, or chalcogenide compounds thereof. The rate of recording is limited by the heat conductivity of the medium.
The Fitzpatrick patent describes a digital writing process in which a film of semiconductor material, such as cadmium telluride, on a substrate of plastic such as methylmethacrylate or polycarbonate, is exposed to a recording laser beam that heats the plastic substrate to produce a pressurized gas bubble that bursts the overlying semiconductor leaving a pit or hole in the reflective surface that represents one bit of information. The rate at which information can be recorded is limited by the amount of heat required to cause the eruption and the heat conductivity of the recording medium.
The Reilly patent describes a recording medium formed by a thin continuous layer of metal in which bits of data are recorded by alterations produced with focused spots of laser light. A transparent dielectric coating is provided to increase the light absorption of the metal layer.
The Komurasaki patent describes a real-time monitor for use with a recording medium comprising a continuous film of metal such as bismuth, gold or chromium which is selectively melted or vaporized by a focused light beam to record one bit of information.
U.S. Pat. No. 4,380,769 to Thomas et al. describes the recording of information by the thermal deformation of a continuous thin film of amorphous material carried by a plastic substrate. Individual depressions surrounded by sharply defined ridges are produced in the amorphous film.
U.S. Pat. No. 4,334,233 to Murakami describes a dust-protecting shield over the substrate that minimizes information distortion that might otherwise occur because of dust particles on the recording medium.
U.S. Pat. No. 4,428,075 to Hazel describes a preformatted disk in which synchronization marks are recorded in areas separate from the data recording areas. These tracking and timing marks are distinct from the alterations that represent bits of data and, to the extent they occupy space that could otherwise be used for digital storage, reduce the capacity of the disk.
The formation of arrays having microscopic relief patterns is known in the photographic field where such techniques are used to reduce variations in image density. U.S. Pat. Nos. 4,366,299 to Land and 4,402,571 to Cowan et al. discuss the formation of spaced discrete holes using a photoresist that is exposed twice to the interference patterns of two laser beams, one exposure being below the threshold for the development of the photoresist. Land also describes for photographic purposes the formation of peaks coated with silver as one step in formation of a silver halide coating. The structure proposed by Land does not lend itself to the recording of digital information.
U.S. Pat. No. 3,019,124 to Rogers discloses a method of manufacturing photographic elements by applying a first light sensitive layer in a uniform thickness to a support, embossing the coated layer to form a relief impression having systematically arranged spaced elevated sections joined by depressed sections interspersed between them, and applying a second light sensitive layer having a different spectral sensitivity to fill the depressions remaining in the surface to the level of raised sections.
U.S. Pat. No. 4,362,806 to Whitmore describes a photographic substrate comprising an array of microvessels that are filled with various photographic materials. The object is to reduce lateral image spreading by providing a discontinuous recording substrate. The microvessels are separated only by minute distances that play no part in the recapture of information. Any appreciable thickness of the walls separating the microvessels detracts from the continuous image that is the object of the Whitmore disclosure. The recording is done over mass areas and the microvessel walls are used to prevent undesired lateral spreading of the photographic image. Whitmore suggests electronically scanning the photographic elements to read information in digital format. Whitmore also discloses modifying the microvessels by scanning with a laser beam to alter the character of selected microvessels by melting, sublimation or change in viscosity. The microvessels of Whitmore require subsequent photographic processing to provide optically readable information.